Narrative:

G1000 with GFC700 autopilot flying with autopilot engaged. Setting up for VOR [approach] with arc and tuning course knob for VOR 1 and VOR 2. While turning course knob the altitude alert went off showing my altitude at 5;600 ft. I started a descent back to 4;000 ft and ATC asked me to say altitude. I advised I was at 4;600 ft descending back to 4;000 ft. I leveled at 4;000 ft and ATC requested me to say altitude; and I reported level at 4;000 ft. They responded that my mode C differs by 2;000 ft and to stop altitude squawk. I cancelled IFR. When cross checking the G1000 altitude with backup altimeter and they differed by almost 2;000 ft. The G1000 showing almost 2;000 ft higher than backup altimeter. Then I observed the barometric setting on the G1000 and it read 32.08 with the local altimeter 30.06. That caused the altitude discrepancy. The course knob and [altimeter barometric correction] knob are located together on the G1000. While turning the course knob I had my thumb and forefinger against the [barometric correction] knob and inadvertently rotated the [altimeter barometric correction] knob to 32.08 which started the above event. More diligent awareness when rotating knobs and a better scan and cross check before doing anything. It would not have happened at all if the [altimeter barometric correction] knob was not combined with any other knob but was a knob all by itself.

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Original NASA ASRS Text

Title: The pilot of a C182 was advised that his altimeter mode C altitude did not agree with his reported altitude. This was confirmed by comparing his G1000 altitude indicator against the back-up altimeter. The pilot then recognized that the altimeter barometric reading in the Kollsman window differed greatly from the local pressure. He surmised that he inadvertently adjusted the co-located barometric correction knob while attempting to adjust the autopilot course knob.

Narrative: G1000 with GFC700 autopilot flying with autopilot engaged. Setting up for VOR [approach] with arc and tuning course knob for VOR 1 and VOR 2. While turning course knob the altitude alert went off showing my altitude at 5;600 FT. I started a descent back to 4;000 FT and ATC asked me to say altitude. I advised I was at 4;600 FT descending back to 4;000 FT. I leveled at 4;000 FT and ATC requested me to say altitude; and I reported level at 4;000 FT. They responded that my mode C differs by 2;000 FT and to stop altitude squawk. I cancelled IFR. When cross checking the G1000 altitude with backup altimeter and they differed by almost 2;000 FT. The G1000 showing almost 2;000 FT higher than backup altimeter. Then I observed the barometric setting on the G1000 and it read 32.08 with the local altimeter 30.06. That caused the altitude discrepancy. The course knob and [altimeter barometric correction] knob are located together on the G1000. While turning the course knob I had my thumb and forefinger against the [barometric correction] knob and inadvertently rotated the [altimeter barometric correction] knob to 32.08 which started the above event. More diligent awareness when rotating knobs and a better scan and cross check before doing anything. It would not have happened at all if the [altimeter barometric correction] knob was not combined with any other knob but was a knob all by itself.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.